The present invention relates to a pneumatically operated disk brake which is provided particularly for commercial road vehicles.
Pneumatically operated disk brakes or pneumatic disk brakes of the above-mentioned type are known, for example, from German Patent Document DE-OS 37 16 202, from German Patent Document DE-OS 40 32 885 as well as from German Patent Document DE-OS 42 12 384 which is no prior publication. In the case of these known disk brakes, a caliper which is disposed to be slidable in the axial direction reaches around a brake disk. On one side of the caliper, a pneumatically operated application device is arranged. When this pneumatically operated application device is actuated, a brake shoe situated on this side of the brake disk is pressed against the corresponding friction surface of the brake disk. As a result of the reaction forces, the caliper will be displaced in the opposite direction and will therefore press a brake shoe which is situated on the opposite side also against the brake disk.
In the case of these known disk brakes, the application device has a rotary lever which operates as an actuating element and which is disposed to be swivellable about an axis of rotation which extends in parallel to the plane of the brake disk. On its side facing the brake disk, the rotary lever rests via an eccentric approximately in the longitudinal center against a traverse member which extends in parallel to the axis of rotation and which is guided to be slidable with respect to the brake disk. Two adjusting spindles with an external thread are screwed in an adjustable manner into a respective assigned internal thread of the traverse member in a parallel arrangement. Because two adjusting spindles are used, an application device of this type is also called a "two-spindle application device".
The two adjusting spindles each act via a pressure piece disposed on their brake-disk-side end on the brake shoe disposed in a slidable manner with respect to the brake disk on the application side in the caliper. The ventilating play, which changes because of the wear of the lining, constantly maintains its correct desired value by at least one adjusting device. The adjusting device is coupled in an axially displaceable manner but rotationally fixed with one of the two adjusting spindles and acts, during each actuating of the rotary lever, via a sliding clutch on the respective adjusting spindle. The rotation of the adjusting spindle which is caused by the adjusting device in the case of a wear of the lining is transmitted by a synchronization device to the second adjusting spindle such that this adjusting spindle is rotated about the same angle and therefore reduces its distance to the brake disk by exactly the same amount.
In the simplest case, only one driving device is arranged in the second adjusting spindle, which driving device couples this adjusting spindle in a rotationally fixed manner with the synchronization device. The driving device may be, for example, a gearwheel which engages in an internal axial toothing of the adjusting spindle, which gearwheel is coupled via a shaft with the synchronization device. However a separate adjusting device may also be provided for the second adjusting spindle. In this case, adjusting forces can be achieved that are twice as high which, however, is connected with correspondingly higher manufacturing costs.
When only one adjusting spindle is present ("single-spindle" application device) the synchronization device is naturally eliminated.
As mentioned above, the precise adjustment of a constantly uniform ventilating play is very important for the secure and reliable functioning of the disk brakes of this type. Although the adjusting devices which were developed by the Assignee of the applicant operate in a reliable and highly precise manner (reference is made in this respect, for example, to German Patent Document DE-OS 40 34 165 corresponding to U.S. Pat. No. 5,353,896 of the Assignee of the applicant) it has been found difficult in practice to maintain the ventilating play in the desired tolerance range. In particular, in practice, an undesirable reduction of the ventilating play will frequently occur so that the brake may even tend to lock.
The invention is therefore based on the object of further developing a pneumatically operated disk brake such that a constantly large ventilating play can always be achieved.
According to the invention, this object is achieved by providing a device which prevents any rotation of the adjusting spindle up to a defined torque.
Detailed tests have shown that the ventilating play changes, despite the correct functioning of the adjusting device, since the respective adjusting spindle, when subjected to extensive shaking stress, has the tendency to automatically or without any corresponding torque admission by the adjusting device, to rotate. The rotation is in the adjusting direction because the adjusting device, as a rule, has a directional clutch which prevents a rotation of the adjusting spindle against the adjusting direction. This undesirable rotational movement of the adjusting spindle or spindles is therefore the cause of the undesirable change of the ventilating play.
The invention therefore suggests that a device be provided which prevents any rotating of the threaded spindle or spindles up to a defined torque. As a result, any shaking stress which may occur in practice will definitely be unable to cause an undesirable rotation of the adjusting spindle or spindles so that the ventilating play will always remain constant. At the same time, when the torque is selected in an appropriate manner, the adjusting device is capable of rotating the adjusting spindle or spindles when there is a wear of the brake lining. In which case, the slightly increased torque which is required for this purpose presents no problems in the given force relationships and drive performances of the adjusting device.
According to the advantageous further development of the invention, the required braking torque for the respective adjusting spindle can be achieved, for example, an elastic element which acts upon the threaded surface of the concerned threaded spindle. The elastic element may, for example, be a friction ring made of rubber or the like which is fastened on the end of the corresponding threaded bore of the traverse member which faces the brake disk or is on the outside and whose inside diameter is smaller than the outside diameter of the threaded spindle. This arrangement is particularly simple and cost-effective, and furthermore has the advantage that an additional sealing is provided for the rust sensitive threaded surfaces of the adjusting spindle and of the traverse member. Naturally the mentioned friction ring may also be provided on the end of the bore which faces away from the brake disk or is on the inside; finally a simultaneous arrangement on the inner and outer end may also be advantageous.
According to the advantageous further development of the invention, the required braking torque for the respective adjusting spindle may as an alternative be achieved by a counternut which is disposed in a rotationally secured manner on the respective threaded spindle and is spring-loaded in the axial direction. This arrangement can also be provided in a very cost-effective manner and, in contrast to the previously explained embodiment, has the advantage that, also after an extended operating time, no replacement will be required so that its maintenance friendliness will be correspondingly high.
As an alternative, it is also possible to achieve the braking torque required for the respective adjusting spindle by a spring arranged in the force transmission path between the adjusting arrangement and the threaded spindle. The spring correspondingly increases the frictional torque of the force transmission path. This spring may, for example, be a coil spring which is wound around the drive shaft of the threaded spindle and which acts in the axial direction on a gearwheel disposed on the end of the drive shaft so that a corresponding braking torque is exercised on the force transmission which prevents the threaded spindle from automatically rotating. By this arrangement a countertorque is continuously exercised on the force transmitting elements of the synchronization device. Because of the achieved prestressing of the force transmitting elements, an increased wear of these elements is avoided during shaking stress.
As another alternative, the coil spring wound around the drive shaft of the threaded spindle may be designed such that it is fastened at its one end to the housing of the application device and at its other end is actuated by the rotary lever. The actuating of the respective end of the coil spring is constructed such that a torque is exercised on the drive shaft only in the released condition of the brake. This arrangement is therefore suitable where the adjusting device and/or the synchronization device are to be subjected in the operation to braking torques which are as low as possible or in the case of which a high efficiency is endeavored.
For a two spindle embodiment which includes a synchronization device for causing the synchronous adjusting rotational movement, the above-mentioned object of the invention may also be achieved in that a device is provided which locks the synchronization device in the released condition of the brake. Also in the event of a strong shaking stress or the like, the locked synchronization device prevents unintentionally rotation of the two adjusting spindles. A change of the ventilating play when the brake is released is therefore excluded under any circumstances.
When the synchronization device is formed essentially by several gearwheels, the endeavored locking of the synchronization device may be achieved, for example, by a disk formed from an elastomer material if care is taken that one of the gearwheels is pressed in the released condition of the brake against this disk in such a manner that the synchronization device is locked. It is possible, for example, to actuate a leaf spring by the rotary lever. The leaf spring presses the concerned gearwheel in the released condition of the brake against the disk.
However, when a type of synchronization device is provided in which a transmission device is used in the form of a link chain or of a toothed belt, it is recommended to cause the desired locking of the synchronization device by providing an element which is actuated by the rotary lever and which is disposed in a slidable manner. This element presses the link chain and the toothed belt in the released condition of the brake against a guide. For this purpose, a pin may be elastically fastened, for example, on the rotary lever. This pin projects into the housing of the transmission device and, when the rotary lever is actuated, releases the slidably disposed element. Thus, it is achieved in a space- saving and reliable manner that the adjusting spindles, which are coupled with a transmission device, do not rotate in an unintended fashion.
According to another independent aspect of the invention each adjusting spindle is provided on its brake--disk-side end with a frontal toothing which engages in a corresponding frontal countertoothing of its pressure piece. When the depth of the teeth of the frontal toothings is selected such that it corresponds to the desired ventilating play of the brake, the concerned adjusting spindle cannot rotate before the ventilating play is larger than this desired value. An unintended adjustment of the adjusting spindle or of the ventilating play during shaking stress is therefore excluded.